I thought the issue was a Joomla 4 issue, but I have the same issue with another site which is a Joomla 3 site. The template is JA Landscape, and the one thing they both have in common is the T4 plugin.
Here is a link to the existing website in Joomla 3, https://gov.perrycountytn.com. This is a JA Landscape template.
Perry County TN Government is a blog page which demonstrates the blog feature is no longer populating the page in two columns.
The website in this ticket I opened was in Joomla 4, https://test.cityofclifton.com. This is a JA Resort template. And City of Clifton dev site is a blog page which also no longer is populating the blog page in 2 columns as set in the template. I double checked my menu item settings in both sites to make sure I had set it for 2 columns. And I had.
So I don't think this is a Joomla issue, but a Joomlart issue since it is happening in both Joomla 3 and Joomla 4 sites. They are different templates, different Joomla versions, but same T4 plugin. Can you ask your team to recheck how they came to the conclusion it was a Joomla 4 issue?
Thanks,
Pat
Screenshot of Joomla 3 site blog page: